e uropean urology xxx (2012) xxx–xxx
TRANSCRIPT
EURURO-4748; No. of Pages 11
Platinum Priority – Review – Benign Prostatic HyperplasiaEditorial by XXX on pp. x–y of this issue
The Mechanism of Action of Phosphodiesterase Type 5 Inhibitors
in the Treatment of Lower Urinary Tract Symptoms Related to
Benign Prostatic Hyperplasia
Franc ois Giuliano a,*, Stefan Uckert b,c, Mario Maggi d, Lori Birder e, Jay Kissel f, Lars Viktrup f
a Neuro-Uro-Andrology Department of Physical Medicine and Rehabilitation, Raymond Poincare Academic Hospital, Garches, Versailles Saint Quentin en
Yvelines University, Garches, France; b Hannover Medical School, Division of Surgery, Department of Urology & Urological Oncology, Hannover, Germany;c Institute for Biochemical Research and Analysis, Urological Research Unit, Barsinghausen, Germany; d Sexual Medicine and Andrology Unit, Department of
Clinical Physiopathology, University of Florence, Florence, Italy; e Department of Medicine and Pharmacology, University of Pittsburgh School of Medicine,
Pittsburgh, PA, USA; f Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, IN, USA
E U R O P E A N U R O L O G Y X X X ( 2 0 1 2 ) X X X – X X X
ava i lable at www.sc iencedirect .com
journa l homepage: www.europea nurology.com
Article info
Article history:
Accepted September 3, 2012Published online ahead ofprint on September 11, 2012
Keywords:
Benign prostatic hyperplasia
Lower urinary tract symptoms
Phosphodiesterase type 5
Phosphodiesterase type 5
inhibitors
Mechanism of action
Abstract
Context: Clinical trials of phosphodiesterase type 5 inhibitors (PDE5-Is) have consis-tently demonstrated a significant reduction in lower urinary tract symptoms (LUTS) andsmall urinary flow rate changes in men with benign prostatic hyperplasia (BPH).Objective: This review presents the proposed mechanisms of action of PDE5-Is in thetreatment of BPH-LUTS focusing on the localization of PDE5 isoenzymes in the pelvicstructures; smooth muscle relaxation in the bladder, prostate, and supporting vascula-ture; increased blood perfusion of the bladder and prostate; and modulation of sensoryimpulses from these organs.Evidence acquisition: Literature describing in vitro, preclinical, or clinical studies ofpathologic processes contributing to LUTS or effects of PDE5 inhibition on the lowerurinary tract (LUT) was selected for review.Evidence synthesis: We objectively assessed and summarized the published data focus-ing on articles published within the past 10 yr. Articles before the time cut-off wereincluded if historically relevant.Conclusions: The PDE5 isoenzymes are highly expressed in the LUT including thebladder, prostate, and their supporting vasculature. In vitro assays have demonstratedPDE5-Is by regulating cyclic guanosine monophosphate (cGMP) degradation and en-hancing the nitric oxide/cGMP signaling pathway to relax human smooth muscle stripsfrom the prostate, bladder, and LUT arteries. In animals characterized by ischemia/hypoxia of the genitourinary tract, treatment with PDE5-Is increases bladder andprostate tissue oxygenation. PDE5-Is have been shown to reduce nonvoiding contrac-tions and bladder afferent nerve firing in decerebrate spinal cord–injured rats, and toreduce mechanosensitive afferent activities of both Ad- and C-fibers in an irritated oroverextended bladder model.
# 2012 European Association of Urology. Published by Elsevier B.V. All rights reserved.
* Corresponding author. Raymond Poincare Hospital & EA 4501, Universite Versailles St Quentin enYvelines, 104 bd Raymond Poincare, Garches 92380, France. Tel. +33 1 47107748;Fax: +33 1 47104443.E-mail address: [email protected] (F. Giuliano).
Please cite this article in press as: Giuliano F, et al. The Mechanism of Action of Phosphodiesterase Type 5 Inhibitors in theTreatment of Lower Urinary Tract Symptoms Related to Benign Prostatic Hyperplasia. Eur Urol (2012), http://dx.doi.org/10.1016/j.eururo.2012.09.006
0302-2838/$ – see back matter # 2012 European Association of Urology. Published by Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.eururo.2012.09.006
EURURO-4748; No. of Pages 11
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1. Introduction
The normal micturition cycle in the male is a complex process
involving the bladder, prostate, and urethra as well as the
pelvic neuronal and vascular networks innervating and
perfusing these organs (Fig. 1). The tone, contractions, and
relaxation of the smooth detrusor muscle in the bladder
and bladder neck, as well as the smooth and striated
sphincters in the urethra, are mediated by a multifaceted
central and peripheral autonomic and somatic neural control
system coordinated in the spinal cord and brain.
The smooth muscle tone in the lower urinary tract (LUT)
is controlled by various adrenergic, cholinergic, and
nonadrenergic noncholinergic neurotransmitters released
from nerve terminals and endogenous factors from vascular
endothelial sources. The nitric oxide (NO)/cyclic guanosine
monophosphate (cGMP)–mediated pathway and related
key enzymes including phosphodiesterase type 5 (PDE5)
have been shown to play a central role in relaxant responses
of LUT tissue.
During the storage of urine, the parasympathetic inner-
vation of the detrusor is inhibited and the urethral sphincter
is contracted preventing involuntary bladder emptying. This
occurs because of (1) the activation of the sympathetic
innervation conveyed by the hypogastric nerves to the
bladder neck and the urethra, and (2) the recruitment of
pudendal motor neurons to the external urethral sphincter.
This guarding reflex is activated by bladder afferents
conveyed by the pelvic nerves with distension of the bladder
producing low-level bladder afferent firing. The bladder
afferents consist of myelinated (Ad) and unmyelinated (C)
axons. The Ad-fibers respond to passive distension and active
contraction and thus convey information about bladder
filling. The C-fibers are considered insensitive to bladder
filling under physiologic conditions and accordingly termed
‘‘silent’’ C-fibers. However, evidence suggests that C-fibers
may become mechanosensitive under pathologic conditions,
providing nociceptive afferents to overdistension, inflam-
mation, or irritation. The urothelium has specialized sensory
and signaling properties to engage in chemical communica-
tion with nerves in the bladder wall. Urothelium can regulate
Artery
Vas cul arsmoothmuscle
cell laye rs
Prosta� c stromalsmoo th
mus cle cell laye rs
PDE5
PDE5
PDE5
P
PD
Fig. 1 – Phosphodiesterase type 5 (PDE5) is
Please cite this article in press as: Giuliano F, et al. The MechanTreatment of Lower Urinary Tract Symptoms Related to Benign Proj.eururo.2012.09.006
the activity of adjacent nerves and thereby trigger local
vascular changes and/or reflex bladder contractions. During
the elimination of urine, intense bladder-afferent firing
activates reflex pathways that pass through the pons. This
stimulates the parasympathetic outflow to the bladder and to
the urethral smooth muscle and inhibits the sympathetic and
pudendal outflow to the urethral outlet [1].
Detrusor overactivity, prostate obstruction, and altered
anatomic structures in and around the LUT and its vascular
supply are important factors for the development of lower
urinary tract symptoms (LUTS). Benign prostatic hyperplasia
(BPH) is a histologic diagnosis characterized by smooth
muscle and epithelial cell proliferation in the prostate
transition zone, leading to nonmalignant prostate enlarge-
ment [2]. Although prostate enlargement due to BPH has long
been associated with LUTS, it is widely recognized that it is
not the exclusive cause. Pathophysiologic risk factors for
LUTS suggestive of BPH (BPH-LUTS) include pelvic reduction
in nitric oxide synthase (NOS)/NO, atherosclerosis/pelvic
ischemia, autonomic overactivity, altered androgen envi-
ronment, and local inflammation [3,4].
Tadalafil for once-daily use, a long-acting PDE5 inhibitor
(PDE5-I), represents the first new class of drug approved by
the US Food and Drug Administration in the past 20 yr for
men with BPH-LUTS and for men with coexisting erectile
dysfunction (ED) and BPH-LUTS. Clinical studies showed
that tadalafil improved symptoms of BPH, including both
storage and voiding symptoms, without the sexual dys-
function side effects seen in other BPH-LUTS treatments
[5–10]. However, peak urinary maximum flow rate (Qmax)
per uroflowmetry, although improved for both tadalafil and
placebo, was typically not statistically different. Because of
the small Qmax changes but the consistent, significant, and
clinically meaningful improvement in urinary symptoms,
questions have arisen about the mechanism of action for a
PDE5-I such as tadalafil in the treatment of BPH-LUTS,
although it is generally recognized that there is poor
correlation between symptoms and Qmax [11].
This review provides an updated and simplified evalua-
tion of the potential mechanism of action (MOA) as it
relates to PDE5 inhibition and the clinical improvement in
Pud end alnerve
Hypog ast ricnerve fibers
Pelvic nervefibers
Detr usor s moo thmuscle cell layer s
DE5
E5
oenzymes in the lower urinary tract.
ism of Action of Phosphodiesterase Type 5 Inhibitors in thestatic Hyperplasia. Eur Urol (2012), http://dx.doi.org/10.1016/
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EURURO-4748; No. of Pages 11
BPH-LUTS focusing on the localization of PDE5 isoenzymes
in the pelvic structures; smooth muscle relaxation in the
bladder, prostate, and supporting vasculature; increased
blood perfusion of the bladder and prostate; and modula-
tion of sensory impulses from these organs.
2. Evidence acquisition
Literature was obtained via Medline searches and from the
individual reviewer’s files. Articles were selected that
describe in vitro, preclinical, or clinical studies of pathologic
processes contributing to LUTS or possible effects of PDE5
inhibition in the LUT. Only studies published in English
were included. Relevant reference lists in the respective
literature were also surveyed. When evaluating the effect of
PDE5-Is on BPH symptom improvement in humans, only
randomized placebo-controlled, double-blind clinical trials
were included (level 1 evidence). Articles published within
the past 10 yr were prioritized; however, older articles were
included if they were of historical clinical significance.
3. Evidence synthesis
3.1. Clinical studies assessing phosphodiesterase type inhibitors
in men with benign prostatic hyperplasia-lower urinary tract
symptoms
Clinical trials of PDE5-Is have consistently shown reduction
in storage and voiding symptoms as assessed by the
Table 1 – Mean changes from baseline to end point in total Internationarate in double-blind randomized, placebo-controlled clinical studies o
Study Duration Treatment n
Tadalafil
McVary et al. [6] 12 wk Placebo 143
Tadalafil 5 mg/20 mg 138
Roehrborn et al. [9] 12 wk Placebo 210
Tadalafil 2.5 mg 208
Tadalafil 5 mg 212
Tadalafil 10 mg 216
Tadalafil 20 mg 208
Porst et al. [8] 12 wk Placebo 164
Tadalafil 5 mg 161
Egerdie et al. [5] 12 wk Placebo 200
Tadalafil 2.5 mg 198
Tadalafil 5 mg 208
Oelke et al. [7] 12 wk Placebo 172
Tadalafil 5 mg 171
Tamsulosin 0.4 mg 165
Sildenafil
McVary et al. [13] 12 wk Placebo 162
Sildenafil 50 or 100 mg 179
Vardenafil
Stief et al. [14] 8 wk Placebo 110
Vardenafil 10 mgc 104
IPSS = International Prostate Symptom Score; Qmax = maximum flow rate; VNR =
a Mean change from baseline to end point.b Change calculated by subtracting results reported at 8 wk from baseline.c Twice daily.d Subscores were reported graphically without actual values.* p < 0.05.
Please cite this article in press as: Giuliano F, et al. The MechanTreatment of Lower Urinary Tract Symptoms Related to Benign Proj.eururo.2012.09.006
International Prostate Symptom Score (IPSS) questionnaire
(Table 1) [5,7,8,12–18]. In several large placebo-controlled
studies with tadalafil, improvement in BPH symptoms was
seen within 1–2 wk [5,7,8], and long-term efficacy was
maintained during a 1-yr uncontrolled study [19]. The
short- and long-term reduction in both storage and voiding
symptoms may suggest a mechanism that involves multiple
areas of the LUT. PDE5-Is with a shorter half-life, such as
sildenafil and vardenafil or the modified released PDE5-I
UK-369003, have also shown improvement in BPH symp-
toms in single randomized placebo-controlled studies;
however, these results have not been reproduced, and the
molecules have not been approved for use in men with
BPH-LUTS [12–14].
Although the symptom improvements observed with
PDE5-Is and a-blockers are similar, changes in Qmax with
PDE5-Is have typically not been significantly different than
placebo (Table 1). Interestingly, in the only large placebo-
controlled study conducted in men with BPH-LUTS with
tadalafil and tamsulosin, a small but significant Qmax change
was reported with both tadalafil and tamsulosin compared
with placebo [7]. The significant but modest changes in
Qmax observed in a-blocker trials were attributed to
relaxation of the bladder neck/prostatic smooth muscle
cell layer [20]. In vitro studies have also found that PDE5-Is
relax the bladder/prostatic smooth muscle cell layers
[21–23], but why this effect does not consistently translate
into significant Qmax changes is unclear. Whether or not
these small changes in Qmax identified with both a-blockers
l Prostate Symptom Score (IPSS), IPSS subscores, and maximum flowf phosphodiesterase type 5 inhibitors
Total IPSSa IPSS storagesubscorea
IPSS voidingsubscorea
Qmax, ml/s
�1.7 �1.0 �0.7 0.9a
�3.8* �2.2* �1.7* 0.5a
�2.3 �1.0 �1.3 1.2a
�3.9* �1.6 �2.2* 1.4a
�4.9* �1.9* �2.9* 1.6a
�5.2* �2.0* �3.1* 1.6a
�5.2* �2.1* �3.1* 2.0a
�3.6 �1.3 �2.3 1.1a
�5.6* �2.3* �3.3* 1.6a
�3.8 �1.6 �2.2 1.2a
�4.6 �1.9 �2.7 1.7*,a
�6.1* �2.5* �3.6* 1.6a
�4.2 �1.6 �2.6 1.2 a
�6.3* �2.2 �4.1* 2.4*,a
�5.7* �2.2 �3.5* 2.2*,a
�1.9 VNRd VNRd 0.2a
�6.3* VNR*,d VNR*,d 0.3a
�3.6 �1.6 �1.9 1.0b
�5.9* �2.7* �3.2* 1.6b
value not reported.
ism of Action of Phosphodiesterase Type 5 Inhibitors in thestatic Hyperplasia. Eur Urol (2012), http://dx.doi.org/10.1016/
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EURURO-4748; No. of Pages 11
and PDE5-Is are clinically meaningful is debatable. Im-
provement in symptoms is poorly correlated to Qmax
changes [11].
To further elucidate the statistically insignificant Qmax
changes, the urodynamic effect of PDE5 inhibition on the
bladder was assessed in a study of 200 men with moderate
to severe BPH-LUTS with or without bladder outlet
obstruction who were enrolled in an invasive and noninva-
sive urodynamic study. Tadalafil once daily showed no
negative impact on bladder function as measured by
detrusor pressure at Qmax or on any other urodynamic
parameter assessed. The study did not proactively enroll
patients with detrusor overactivity and therefore assessed
only the incidence of involuntary detrusor contractions
and volume at first contraction [24]. The impact of
tadalafil on detrusor overactivity remains to be further
investigated.
Because PDE5-Is significantly improve both BPH-LUTS
and ED, it has been hypothesized that the improvement in
BPH-LUTS is due to ED improvement and changes in the
patient’s quality of life. Several analyses have addressed
whether BPH symptom improvement in clinical studies is
correlated with ED symptom improvement. In a post hoc
analysis from a dose-ranging tadalafil study in 716 men
with ED and 340 men without, changes in BPH-LUTS after
12 wk of treatment with placebo or various doses of once-
daily tadalafil were similar in men with or without
comorbid ED, suggesting the improvement in BPH-LUTS
was also independent of ED changes [25]. These finding
were confirmed in another tadalafil study [8]. Therefore,
although the mechanism by which PDE5-Is improve
BPH-LUTS may share similar pathways in which PDE5-Is
improve ED, these are independent of each other.
The consistent improvement in total IPSS and IPSS
subscores across all PDE5 clinical studies confirm the
validity of PDE5-Is as an important new class for treating
BPH-LUTS. The following sections highlight the multiple
mechanisms by which PDE5-Is may have an impact on the
pathophysiology of BPH-LUTS.
3.2. Phosphodiesterase type 5 localization in the human
outflow region (bladder, prostate, urethra)
3.2.1. Urinary bladder
Based on the hypothesis that the NO/cGMP signal pathway
may play a role in the mechanism of micturition, the
modulation of intracellular pathways mediated by the
production of cGMP has been suggested to offer a promising
possibility to achieve selective modulation of smooth
musculature of the human urinary bladder. Using chro-
matographic methods, Truss et al. in 1996 were the first to
report the presence of PDE5 in the human detrusor [26].
Immunolabeling for PDE5 was seen in smooth muscle fibers
and also localized in the endothelium and the smooth
muscle layer of the vesicular-deferential arteries (originat-
ing from the inferior vesical artery, the main source of blood
supply to the bladder), maintaining continuous blood
perfusion of the detrusor wall (Fig. 2). Given the ubiquitous
expression of PDE5 in the vascular system, its presence in
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the bladder vascular system was expected. In contrast, the
expression of PDE5 in the urothelium was only sparse [27].
At the messenger RNA level, the expression of PDE5 was also
verified by reverse transcriptase polymerase chain reaction
(RT-PCR) analysis [27–29]. The expression of mRNA
encoding for PDE5 was higher in the detrusor and penile
erectile tissue (corpus cavernosum) than in the prostate
[27,29]. However, despite these findings, a pivotal role for
NO and cGMP-mediated signals in the control of detrusor
smooth muscle has yet to be established. Cyclic adenosine
monophosphate (cAMP) regulated by PDE type 4 isoen-
zymes may play an even larger role in bladder smooth
muscle cell relaxation.
3.2.2. Prostate
There is evidence from numerous experimental studies that
the NO/cGMP system and related key proteins, including
the cGMP-degrading PDE5, are pivotal players in the control
of the normal function of the prostate. This may include the
contractile activity of the smooth musculature, secretory
glandular function, as well as the regulation of proliferation
of smooth muscle, glandular epithelial cells, and stromal
connective tissue [30,31]. Kuciel and Ostrowski in 1970
were the first to isolate the activity of phosphodiesterase
enzymes from human prostate tissue [21]. Using ion
exchange chromatography to separate proteins and an
assay based on tritium-labeled cGMP, PDE5 was detected in
cytosolic supernatants from minced human prostate tissue
excised from the transition zone [22]. The expression of
mRNA encoding for PDE5 in the prostate was later
confirmed by quantitative RT-PCR [29]. However, these
research approaches did not provide sufficient information
on the localization of PDE5 in the prostate.
The distribution of PDE5 in different histologic portions
of the prostate was revealed by immunohistochemistry:
Utilization of specific antibodies demonstrated the locali-
zation of this cGMP PDE isoenzyme in glandular areas [25],
the smooth musculature of the prostatic stroma, and also
in blood vessels transversing the tissue sections [20,25]. As
shown in Figure 2, prominent localization of PDE5 was
detected in vascular (endothelial and smooth muscle) cells
of human prostate. It was also shown that, in the transition
zone of the prostate, PDE5 is localized in close conjunction
to other key mediators of the NO/cGMP pathway. For
example, in the smooth musculature, the PDE isoenzyme
was found colocalized with its main substrate cGMP. The
PDE5/cGMP-positive smooth muscle bundles were seen
transversed by slender varicose nerve fibers immunoreac-
tive for the neuronal isoform of NOS (nNOS). The smooth
muscle fibers also presented abundant staining for the
cGMP-binding, cGMP-dependent protein kinase cGKI
(here cGKIß). Interestingly, abundant labeling specific
for the cyclic AMP-binding protein kinase A was also
registered in bundles of PDE5-immunoreactive smooth
musculature. These bundles were innervated by nerve
fibers containing significant amounts of vasoactive intes-
tinal polypeptide (VIP), a neuropeptide known to promote
the local production of the second messenger molecule,
cAMP [32].
ism of Action of Phosphodiesterase Type 5 Inhibitors in thestatic Hyperplasia. Eur Urol (2012), http://dx.doi.org/10.1016/
Fig. 2 – Phosphodiesterase type 5 (PDE5) expression and immunolocalization in human tissues. (a) An intense PDE5 immunopositivity was detected in thesmooth muscle bundles and endothelial layer surrounding the vascular bed of transverse sections of human deferential artery. (b) Representativenegative control image, hematoxylin counterstained, obtained by omitting the primary anti-PDE5 antibody in a transverse section of human deferentialartery. (c) The prostatic gland section shows a scanty PDE5 immunostaining in fibromuscular stroma (asterisks), whereas it is mainly distributed in theendothelial and smooth muscle cells of blood vessels (arrows). (d) An intense PDE5 immunostaining was detected in both smooth muscle and endothelialcomponent of corpora cavernosa section. Magnification T4. Reproduced with permission from the International Society for Sexual Medicine [37].
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3.2.3. Urethra
It is well established that the urethra is pivotal in
maintaining urinary continence and enabling coordinated
micturition in both genders. Up to the present, only a very
few studies have addressed the mechanisms controlling the
function of human urethral smooth musculature. While
the contraction of urethral smooth muscle mediated by the
activation of a-adrenoreceptors has been attributed to the
continence mechanism, the relaxation of the longitudinal
and/or circular smooth muscle layer during micturition has
been assumed to be mediated by the NO/cGMP pathway.
The functional significance of the PDE5 and other key
proteins of the cGMP signaling in this process has not
been clarified comprehensively. Immunohistochemistry
performed on sections of human female urethra demon-
strated the expression of the cGMP-specific PDE5 in vascular
and nonvascular smooth muscle cells and in the vascular
endothelium. In the nonvascular smooth muscle, PDE5 was
found colocalized with the cGMP-binding protein kinase
cGKI, whereas in vascular endothelial cells, co-staining for
PDE5 and cGMP was seen [33]. More recently, the predomi-
nant expression of mRNA transcripts specifically encoding
for PDE5A (cGMP-PDE) was shown by means of RT-PCR
analysis [34]. Another molecular biology approach that
specifically investigated the expression of the cGMP PDE5 in
human LUT tissue found a consistent expression of the
enzyme in the prostatic urethra. Here, the abundance of
expression was higher than in the prostate gland [29]. These
findings were paralleled by immunohistochemical investi-
gations to describe the localization of PDE isoenzymes in the
human male distal (penile) urethra.
In the tissue sections, PDE5-immunoreactive smooth
muscle bundles were seen innervated by varicose nerve
fibers characterized by the expression of nNOS; some of
these nerves also presented staining specific for the VIP and
calcitonin gene-related peptide [34]. Although the striated
musculature seems to play a role in urethral function, no
study has yet addressed the expression of PDE5 in human
tissue. Using rat tissue, the expression and distribution of
PDE5 was shown in striated muscle of the urethra, where it
was predominantly seen colocalized to the Z-band stria-
tions. The amount of PDE5 in the striated component was
six times that observed in the smooth musculature,
suggesting that PDE5 is possibly significant in the regula-
tion of striated muscles [27]. However, it remains to be
clarified whether these findings can be replicated in
humans and what the functional implications are.
3.3. Smooth muscle relaxation
The issue of how an enhancement of the cGMP pathway—
for example, by inhibiting the activity of PDE5—can affect
the relaxation of LUT smooth musculature has been
investigated in various in vitro tissue bath studies typically
using nonmalignant isolated surgical specimens of the
urinary bladder (including both the detrusor and bladder
neck), prostate, or urethra. The experimental models have
applied agents such as muscarinic (carbachol) or adrenergic
receptor agonists (norepinephrine, phenylephrine) known
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to contract effectively the respective tissues, as well as
various PDE5-Is and the NO donor drug sodium nitroprus-
side (SNP), characterized as a reference drug to stimulate
the production of cGMP via activation of the enzyme
guanylyl cyclase.
3.3.1. Urinary bladder
When considering the effect of PDE5-Is on the human
urinary bladder, it is important to differentiate between
bladder dome and bladder neck smooth musculature.
Regulation of human bladder smooth muscle by the NO/
cGMP pathway differs markedly according to the region of
the bladder studied. Early studies using PDE5-Is and square-
shaped strips of human detrusor smooth muscle (full wall
specimens devoid of the urothelium) challenged by the
muscarinic agonist carbachol did not present striking
evidence supporting a role of PDE5 in the control of bladder
function: The relaxant responses of the tissue to the
cumulative addition (0.01–200 mM) of the PDE5-Is zapri-
nast and dipyridamole were determined as <20% [26]. Oger
et al. investigated the effect of sildenafil (10 nM–30 mM) on
the tonic contraction of human bladder dome smooth
muscle in response to carbachol [28]. Sildenafil exerted a
direct relaxant effect; however, high concentrations of
the PDE5-I were needed. The relaxant effect involves the
cGMP pathway as well as the activity of K+ channels.
Because the relaxation remained unaltered in the presence
of SNP, it was assumed that NO only makes a minor
contribution to the relaxation induced by sildenafil [35].
Thus, in the human bladder dome, the effects of PDE5-Is are
moderate and may not completely explain the improve-
ment of urinary storage symptoms observed in patients
treated with PDE5-Is.
In the human bladder neck, the nitrergic innervation is
more prominent, and thus the effects of PDE5-Is are
different. In isolated human (male) bladder neck strips,
SNP induced a mediocre relaxation (maximum: 37% � 4%)
of preparations precontracted with carbachol. This relaxation
increased significantly (to 62% � 3%) following preexposure of
the tissue to a threshold concentration of the PDE5-I
vardenafil (100 nM) [27]. Similar findings were reported with
sildenafil where a relaxing effect was shown on isolated
human bladder neck precontracted with phenylephrine [36].
It was also shown in experiments using human vesicular-
deferential arteries that tadalafil increased the relaxant
response to SNP [37].
3.3.2. Prostate
Uckert et al. conducted tissue bath studies using smooth
muscle isolated from the periurethral and transition zones
of nondiseased human prostates [22]. A dose-dependent
reversal was seen of the tension induced by norepinephrine
in response to sildenafil and zaprinast (1 nM–10 mM) [23].
However, the efficacy of these compounds did not exceed
30% reversal of the initial contractile force generated by the
tissue preparations in response to norepinephrine. In
another experimental sequence using the same in vitro
setup, the contraction of the tissue was also antagonized by
tadalafil (mean: �52% reversal of tension) and vardenafil
ism of Action of Phosphodiesterase Type 5 Inhibitors in thestatic Hyperplasia. Eur Urol (2012), http://dx.doi.org/10.1016/
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EURURO-4748; No. of Pages 11
(mean: �35%) [23]. PDE5-Is might also interfere with the
contraction of prostatic smooth muscle mediated by the
release of endogenous peptides because tadalafil could also
reduce prostate contractions induced by endothelin-1 [38].
It was also shown that PDE5-Is tend to be more effective
in vitro in the presence of a threshold concentration of
sodium nitroprusside known to stimulate the activity of the
soluble guanylyl cyclase (sGC) [39]. This is due to a
synergistic effect that is likely to result from combining
both an enhanced local tissue production of cGMP by NO via
the sGC and blockade of the breakdown of the second
messenger by PDE5-Is.
3.3.3. Urethra
In the human urethra, large amounts of NOS-containing
nerves have been demonstrated in the muscular wall,
around blood vessels close to the urothelium, as well as in
the sarcolemma of intramural striated muscle fibers of the
membranous urethra [40]. PDE5-immunoreactive smooth
muscle bundles innervated by varicose nerve fibers
characterized by the expression of nNOS, the calcitonin
gene-related peptide, and VIP were also seen [34]. In
preliminary organ bath experiments, the contraction
induced by noradrenaline of isolated human female urethra
was almost completely reversed in response to 10 mM
sildenafil; the reversal of the tension brought about by
tadalafil (30 mM) was 47% [33]. In a similar experimental
setup using specimens of male proximal penile urethra, the
adrenergic tension of the tissue was antagonized by 35%,
26%, and 20% following the application of 10 mM sildenafil,
vardenafil, or tadalafil, respectively. The relaxation effects
were paralleled by a several-fold elevation in cGMP [41]. An
inhibition of the contraction induced by phenylephrine of
muscle strips of the prostatic urethra obtained from male
New Zealand rabbits in response to the PDE5-I udenafil was
also shown. Udenafil relaxed the urethral strip preparations
in a dose-dependent manner, with a maximum relaxation at
the final drug concentration (1 mM) of 44% [42]. In contrast,
in another study, the maximum contraction response
obtained through electrical field stimulation of circular
segments of guinea pig urethra was not altered by pretreat-
ment with a high concentration of SNP (100 mM). In tissue
specimens challenged by noradrenaline, the relaxation
observed at 10 Hz was not attenuated in the presence of
the NO synthase inhibitor NG-nitro-L-arginine. Thus it
remains unclear whether the relaxation of urethral smooth
muscle depends on the activity of NO and cGMP [43].
3.4. Increased blood perfusion
A growing body of epidemiological studies documented a
strong and independent relationship between BPH-LUTS and
metabolic syndrome, which is essentially characterized by a
syndromic clustering of cardiovascular and metabolic altera-
tions [44]. Research in animal models of LUTS indicated that
either systemic hypertension [37,45] or metabolic derange-
ment [46–48] can lead to morphologic/structural alterations
of both prostate and bladder including fibrosis, increased
contractile activity, and urethral resistance, having chronic
Please cite this article in press as: Giuliano F, et al. The MechanTreatment of Lower Urinary Tract Symptoms Related to Benign Proj.eururo.2012.09.006
ischemia as a common feature. Hence the view that pelvic
ischemia/hypoxia might underlie LUTS is emerging. Blood
supply to the LUT (lower part of the bladder, the prostate, and
the seminal vesicles) is provided by small branches of the
inferior vesical artery, which frequently arises in common
with the middle rectal artery from an anterior division of the
internal iliac artery [49]. Interestingly, the human vesical
deferential artery is enriched in PDE5, showing mRNA levels
and cGMP hydrolyzing activity comparable with corpora
cavernosa [37]. In addition, blood vessels within the prostate
and bladder are widely positive for PDE5 expression, localized
in the endothelial and smooth muscle cells [29]. Pelvic
vasculature might therefore represent a new target for PDE5-
Is. Accordingly, in a rat experimental model, it was recently
demonstrated that PDE5 actively regulates blood supply to
the LUT [37,45]. Spontaneous hypertensive rat (SHR) is a rat
strain characterized by a reduced pelvic blood flow to the
genitourinary tract when compared with its normotensive
counterpart, Wistar-Kyoto rats (WKY) [50]. By injecting these
rats intraperitoneally with the bioreductive drug pimonida-
zole hydrochloride (Hypoxyprobe), it is possible to visualize
hypoxic cells within LUT by immunohistochemistry. Hypox-
yprobe is a water-soluble substituted 2-nitrominidazole that
forms adducts with proteins in cells that are at an oxygen
pressure �10 mm Hg. Prostate and bladder of SHR rats
are definitively more hypoxic than in WKY rats, with a
predominant distribution of hypoxic cells in the epithelial
layers. Both acute (vardenafil 10 mg/kg, 90 min before death;
see Fig. 3: rat bladder from Morelli et al. [45]) and chronic
(tadalafil 2 mg/kg per day for 1, 7, and 28 d; see Fig. 4: rat
prostate from Morelli et al. [37]) PDE5 blockade restored LUT
oxygenation in SHR rats up to the level observed in WKY rats.
A 2009 study, performed using contrast-enhanced ultrasound
in 12 BPH patients awaiting surgery, demonstrated increased
prostatic blood perfusion following administration of 20 mg
tadalafil [51]. These preliminary findings indicate that PDE5-
Is might relax LUT vasculature and consequently increase
blood supply and tissue oxygenation, therefore possibly
ameliorating urinary function and reduce BPH-related LUTS.
The fact that LUTS are significantly reduced within 1–2 wk in
clinical trials suggests that other factors than increased blood
perfusion play a role.
3.5. Afferent nerve activity
The NO/cGMP pathways are believed to play important
roles in the function of the nervous system in the LUT. nNOS
is expressed in locations including the uroepithelium and
nerves innervating the bladder neck and urethra; endothe-
lial NOS is present in vascular endothelium [52,53].
Enzymatic activity (NOS) has been measured from various
regions including the bladder neck and prostatic urethra
[54]. Guanylate cyclase has been detected in afferent nerves
and interstitial cells with the highest levels found in the
urethra. Increases in interstitial cell number and connec-
tivity have been demonstrated in animals following
experimental spinal cord injury (SCI) [55,56], which results
in augmented pacemaker activity in these cells. This type of
activity, in turn, may drive intrinsic smooth muscle
ism of Action of Phosphodiesterase Type 5 Inhibitors in thestatic Hyperplasia. Eur Urol (2012), http://dx.doi.org/10.1016/
Fig. 3 – Immunohistochemical staining of Hypoxyprobe in rat bladder. Hypoxyprobe-positive protein adducts were revealed in hypoxic cells (PO2 <10 mmHg) of bladder transverse sections by a monoclonal antibody (magnification T10). (a) Wistar-Kyoto rats (WKY): Only scanty positive labeling is present.(b) Untreated spontaneously hypertensive rats (SHR): Massive hypoxia is present in both the urothelium/suburothelium (arrows) and vascularendothelium (arrowheads). (c) Vardenafil-treated spontaneously hypertensive rats (SHR). Hypoxyprobe labeling is dramatically decreased; only a fewendothelial (arrows) cells are positive. Reproduced with permission from the International Society for Sexual Medicine [45]. WKY = Wistar-Kyoto rats.
Fig. 4 – Immunohistochemical staining of Hypoxyprobe in rat prostate. Hypoxyprobe-positive protein adducts were revealed in hypoxic cells (PO2 <10 mmHg) of prostate transverse sections by a monoclonal antibody (magnification T10). Images (magnification T10; bars = 50 mm) are representative of eachexperimental group at each time point (a–c: 1 d; d–f: 7 d; g–i: 28 d). The staining was almost undetectable in WKY prostate sections (a, d, g). The prostatesections from untreated SHR were strongly immunopositive for Hypoxyprobe in the epithelial layers (arrows) surrounding prostate ducts, which werealso characterized by alveolus dilation and reduction of interstitial and stromal spaces (b, e, h). Prostate sections from tadalafil-treated SHR show a netreduction of Hypoxyprobe immunopositivity after (c) 1 d, becoming absent after both (f) 7 d and (i) 28 d. Reproduced with permission from theInternational Society for Sexual Medicine [37].
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contractions, thereby stimulating afferent firing [57]. Beside
afferent nerves, the urothelium (and also urethral epitheli-
um) are able to synthesize and release NO in response to a
number of stimuli [58,59]. Release of NO may in part act to
uncouple cells (urothelial, interstitial) preventing this type
of pacemaker activity.
PDE5-Is are thought to decrease the perception of
bladder filling, thereby reducing the sensation of urgency.
The mechanism may involve a reduction in the release of
neuropeptides and activity of afferent nerves via activation
of the NO/cGMP pathway [60,61]. In the urinary bladder, the
NO/cGMP pathway can have a number of functional roles.
Both the production of NO as well as the expression levels of
enzymes that synthesize NO can be altered by a variety of
manipulations and/or pathophysiologic mechanisms. For
example, inhibition of NO can result in bladder hyperactiv-
ity and decrease in bladder capacity [62]. Chronic NO
deficiency [63] or scavenging of NO via use of intravesical
oxyhemoglobin [64] has been shown to result in bladder
overactivity or hyperactivity to various stimuli. This finding
may be due to changes in levels of NO acting at a number of
sites including bladder afferents, interstitial cells, as well as
smooth muscle, resulting in alterations in bladder activity.
Only a limited number of studies address the impact of
PDE5 inhibition in sensory function, and differences
between species and the influence of pathology can make
interpretation difficult. The impact on bladder hyperactivity
was assessed by Caremel and colleagues in anesthetized
female Sprague-Dawley rats that were continuously per-
fused with capsaicin and then challenged with an NO donor
(SNP), a cGMP analog (8Br-cGMP), or a PDE5-I (either
sildenafil or vardenafil) [65]. The addition of NO or PDE5-Is
increased the intercontraction interval and micturition
pressure threshold, suggesting that the NO/cGMP signaling
pathway exerted an inhibitory effect on bladder afferent
activity [65]. Minagawa and colleagues recently examined
the impact of increasing doses of intravenous tadalafil on
bladder afferent nerve activity caused by bladder distension
and acrolein-induced hyperactivity in female Sprague-
Dawley rats [66]. Bladder nerve afferents were identified
by electrical stimulation of the pelvic nerve and by bladder
distension, and divided by conduction velocity into
myelinated Ad- or unmyelinated C-fibers. Tadalafil signifi-
cantly decreased the single afferent activity of both the Ad-
and C-fibers in response to bladder filling and urothelial
bladder-installation irritation without affecting the bladder
tone [66]. SCI rats mimic the voiding pattern of patients
with neurogenic detrusor hyperactivity due to SCI by
displaying nonvoiding contractions (NVCs) during bladder
filling. NVCs in SCI rats are associated with bladder afferent
fiber hyperexcitability [67,68].
When Behr-Roussell and colleagues examined the
impact of PDE5 inhibition with vardenafil in SCI rats, they
found a significant reduction in bladder afferent nerve firing
during bladder filling and a significant decrease in ampli-
tude of NVCs [69]. In a similar animal model, Sasatomi and
colleagues found that increased NO level caused by the
inhibition of arginase, which degrades L-arginine into L-
ornithine, decreased NVC through reduced afferent nerve
Please cite this article in press as: Giuliano F, et al. The MechanTreatment of Lower Urinary Tract Symptoms Related to Benign Proj.eururo.2012.09.006
activity [70]. Taken together, these studies show support for
NO/cGMP pathway and a MOA for PDE5 inhibition in the
reduction of afferent nerve activity in both bladder
physiology and pathophysiology.
4. Conclusions
PDE5 isoenzymes are highly expressed in human LUT tissues.
PDE5 inhibition results in smooth muscle relaxation and
increased pelvic blood perfusion in these tissues and likely
modulates afferent nerve activity. This activity may affect
nonvascular or vascular smooth muscle tone. We propose
that the improvement in both storage and voiding urinary
symptoms observed after 1–2 wk of tadalafil could be caused
by the smooth muscle cell relaxation in bladder neck,
prostate, and urethra, with the maintenance of effect possibly
supported by the smooth muscle cell relaxation of these
organs’ vascular supply and increased blood perfusion and
oxygenation. Modulation of the sensory output from the LUT
is likely to play a role in both the short and long term.
Author contributions: Franc ois Giuliano had full access to all the data in
the study and takes responsibility for the integrity of the data and the
accuracy of the data analysis.
Study concept and design: Viktrup, Kissel.
Acquisition of data: Giuliano, Uckert, Maggi, Birder, Kissel, Viktrup.
Analysis and interpretation of data: Giuliano, Uckert, Maggi, Birder, Kissel,
Viktrup.
Drafting of the manuscript: Giuliano, Uckert, Maggi, Birder, Kissel,
Viktrup.
Critical revision of the manuscript for important intellectual content:
Giuliano, Uckert, Maggi, Birder, Kissel, Viktrup.
Statistical analysis: None.
Obtaining funding: Viktrup.
Administrative, technical, or material support: Kissel.
Supervision: None.
Other (specify): None.
Financial disclosures: Franc ois Giuliano certifies that all conflicts of
interest, including specific financial interests and relationships and
affiliations relevant to the subject matter or materials discussed in the
manuscript (eg, employment/affiliation, grants or funding, consultan-
cies, honoraria, stock ownership or options, expert testimony, royalties,
or patents filed, received, or pending), are the following: Franc ois
Giuliano is a consultant and lecturer for Eli Lilly and Company and a
consultant and investigator for Bayer-Schering. Jay Kissel and Lars
Viktrup are employees and stockholders of Eli Lilly and Company. The
other authors have nothing to disclose.
Funding/Support and role of the sponsor: Eli Lilly and Company helped
interpret the data and prepare, review, and approve the manuscript.
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